Lord of the rings

The bacterium Deinococcus radiodurans is able to resist doses of ionizing radiation many times higher than those that kill other organisms. In the January 10 Science, Smadar Levin-Zaidman and colleagues at the Weizmann Institute of Science, Israel, report that the D. radiodurans genome exists in an unusual ring-like structure that may account for its radioresistance, by restricting the diffusion of radiation-generated free DNA ends (Science, 299:254-256, January 10, 2003).

Scanning electron microscopy analysis revealed that D. radiodurans cells have a tetrad morphology with each quarter containing equal amounts of DNA (each contains a single copy of the bacterial genome). This compartmentalization suggests that DNA repair after radiation does not involve homologous recombination. The bacterial nucleoids adopted a toroidal morphology that presumably dictates a rigid structure facilitating template-independent, error-free, end-joining of DNA breaks.